Abstract

Neisseria meningitidis contains c-type and b-type cytochromes. Oxidation of c-type and b-type cytochromes by oxygen was observed in both cells grown under aerobic and denitrifying conditions, whereas oxidation of cytochromes by nitrite was only seen in cells grown under denitrifying conditions, and the predominant oxidizable cytochromes were b-type. These are likely to be associated with the oxidation of a b-haem containing nitric oxide reductase. Nitrite inhibits the oxidation of cytochromes by oxygen in a nitrite reductase-independent manner, indicating that nitrite may inhibit oxidase activity directly, as well as via the intermediate of denitrification, nitric oxide.
Cytochromes c4 and c2 are major electron donors to the cbb3 oxidase. Both strains deficient in cytochrome c4 and c2 exhibits a growth defect under high level of oxygen. These growth defects are linked to their decreased oxygen consumption rate. The growth defect and the decreased oxygen consumption rate indicated that cytochrome c4 dominates electron transfer to cbb3 oxidase.
Cytochrome c5 is an important electron donor to AniA nitrite reductase. A strain deficient in cytochrome c5 exhibits a growth defect under microaerobic conditions in the presence of nitrite. The mutant can not reduce nitrite to nitric oxide although AniA is expressed normally. A growth defect during high cell density culture under aerobic conditions suggests that cytochrome c5 is also an electron donor to ccb3 oxidase but to a lesser extents than c4 or c2.
Lipid-modified azurin (Laz) was heterologously expressed and purified. The purified protein contains copper ion and can be oxidized or reduced. When oxidized, Laz exhibits an intense blue colour and absorbs visible light around 626 nm. Laz can be oxidized by membrane extract in the presence of oxygen or nitrite. This is likely to be due to the activity of cbb3 oxidase and AniA nitrite reductase, respectively, in membrane extract. However the oxidation rate is very slow. A strain deficient in laz exhibits a growth defect during high cell density culture, similarly to that of c5 mutant. This suggests that Laz might be an electron donor to cbb3 oxidase.